Importance of integrated management of glyphosate-resistant weeds

Integrated weed management (IWM) can be defined as the selection and integration of control methods and involves a set of criteria for their use, so that the results are favorable from an agronomic, economic, ecological and social point of view. In this context, it is essential to be effective in the selection and use of control methods, observing the importance of each one in the management planning to be adopted on the property.

The main methods available to farmers are preventive (care in purchasing seeds, cleaning of machinery and equipment, control on roadsides and paths, etc.), cultural (crop rotation, soil cover, crop management, etc.), mechanical (return weeding, mowing, etc.) and chemical (rotation and diversification of herbicides). Regarding chemical control, more specifically in the grain production system, one issue is gaining increasing importance: the increase in cases of weed resistance to herbicides.

Resistance is the ability of weeds to survive the application of a herbicide to which the same population was previously susceptible. It is a natural occurrence due to the evolution and adaptation of species to environmental changes and the use of agricultural practices, especially the continued application of the same herbicide or herbicides with the same mechanism of action. This situation has occurred with the herbicide glyphosate in grain production areas, especially in the soybean/corn system, where glyphosate is used almost exclusively for chemical weed control.

This practice has resulted in strong selection pressure for herbicide-resistant biotypes. In Brazil, resistance has already been reported in nine weed species (Heap, 2020): caruru-palmeri (Amaranthus palmeri), the giant caruru (Amaranthus hybrydus), white grass (Chloris elata), three species of buva (conyza bonariensis, Canadian Conyza e conyza sumatrensis), bittergrass (Digitaria insularis), the crow's foot grass (Eleusine indica) and ryegrass (Annual ryegrass). Recently, suspected glyphosate resistance in a new species was reported, Euphorbia heterophylla, in the Ivaí Valley region, in the state of Paraná.

This species, popularly known as milkman, milkmaid or wild peanut, originates from tropical regions, being found mainly in Latin American countries and the South of the United States. It is an annual herbaceous plant, with the capacity to complete up to four cycles per year, and can reach up to 1,5m in height. It has relatively large leaves, up to 10cm long, distributed alternately on the stem. This species reproduces exclusively through seeds and can produce up to three thousand seeds per plant (Wilson, 1981). 

Among the main characteristics that make this species stand out as a weed are the high leaf area index and the ability to elongate the stems, making the plant an excellent competitor for light (Willard et al, 1994).

The ideal temperature for seed germination varies between 25°C and 35°C and the presence of light is not necessary for germination to occur (Suda and Giorgini, 2000). The seeds are relatively large, between 2mm and 4mm, which allows germination even at greater depths in the soil, as well as in the presence of straw, characteristic of the direct planting system. 

The presence of dairy farming competing with soybeans can cause losses of 5,1kg/ha per day of coexistence, with the critical period for the start of control being estimated at 17 days after sowing (Meschede et al, 2002). Older studies have already demonstrated that, if no control measures are carried out, productivity losses for soybeans can reach 47% (Chemale and Fleck, 1982). 

The genetic variability found in populations of E. heterophylla is considered high compared to other weeds (Frigo et al, 2009). This means that plants in the same area have very different genetic characteristics, a factor that favors the adaptation of the species in different agricultural systems and, above all, increases the chances of selecting biotypes resistant to herbicides. 

In the past, notably before the introduction of soybeans genetically modified for resistance to the herbicide glyphosate, this weed was considered one of the main pests of soybean crops, not to mention the most difficult for soybean farmers to control. This occurred because, in addition to the specificities of the species, from the 1990s onwards, populations resistant to the group of herbicides with the main mechanism of action used at the time, acetolactate synthase (ALS) inhibitors, were selected, which made the control of this weed even more difficult and complicated for Brazilian soybean farmers (Gazziero et al, 1998). 

With the start of glyphosate-resistant soybean cultivation in Brazil, controlling E. heterophylla populations has become much easier, as this herbicide is now applied two to three times post-emergence. This situation also occurred on the property where the report of milkweed escaping from glyphosate control emerged, in the Ivaí Valley region, Paraná, since the 2008/2009 harvest.

Starting in the 2017/2018 harvest, the producer observed the survival of some plants of this species after glyphosate applications at the recommended doses. In the 2018/2019 harvest, the technical team at Cocari - Cooperativa Agropecuária e Industrial, responsible for providing technical assistance to this farmer, informed the weed team at Embrapa Soja about the situation, and from then on, work was initiated to evaluate the case, following the protocol for reporting cases of weed resistance to herbicides, proposed by the Resistance Committee of the Brazilian Society of Plant Science and the Herbicide Resistance Action Committee of Brazil (HRAC-BR).

First, a new application of glyphosate was carried out in the suspected area, with the recommended dose and application technology, and the plants E. heterophylla were again not controlled by glyphosate. Subsequently, seeds of the surviving biotypes in the field were collected and taken to Embrapa Soja, in Londrina, Paraná, where the first experiment was carried out in a greenhouse, using the dose-response scheme for the herbicide glyphosate.

Subsequently, a second experiment was carried out with the seeds of the surviving plants of this initial experiment (F2 generation), and a third study with the seeds collected from the second experiment (F3 generation), always comparing the suspected population with a population susceptible to glyphosate, whose studied doses of the herbicide were (g ea/ha): 0, 130, 270, 570, 720, 1.080 (standard), 1.440, 2.160, 4.320, 8.640, 17.280, applied at the four true leaf stage. 

The results confirmed the suspicion that the population is resistant to glyphosate. The resistance factor was 4,74 for the control dose of 50% of the suspected population in relation to the susceptible (LD50) and 3,30 for the reduction in the biomass of the suspected population in relation to the susceptible (GR50). 

In addition to the work to verify resistance, studies were carried out on management alternatives for this species, both in a greenhouse and in the field. Monitoring, mitigation and management actions for this new case were also discussed with Cocari's technical management, responsible for technical assistance in the area and partner in the work, in order to keep this problem of resistance restricted to this single area, therefore avoiding , dissemination. In addition, studies on the identification of the resistance mechanism are being carried out in conjunction with the State University of Maringá and Colorado State University. 

The discovery of this new case of resistance reinforces the certainty that, in any agricultural production system, prevention and solution to the problem must always be based on the basic concepts of integrated weed management.

By Fernando Storniolo Wineries,  Dionysius Luiz Pisa Gazziero (Embrapa Soybean); Rubem Silverio Junior, Rafael Romero Mendes (State University of Maringá); Luciano Junior Rodrigues (Cocary)